The mammalian FGF family comprises 18 structurally-related polypeptides, which have traditionally been known as paracrine acting mitogens and morphogens that mediate key processes throughout embryonic development. The discovery that three members of this family act as hormones has transformed the FGF signaling field and sparked major drug discovery activities centered on these ligands in both academia and pharmaceutical industries. FGF21 has been studied as an anti-diabetic drug candidate owing to its glucose- and lipid-lowering action, and various FGF21 agonists are already being tested in clinical trials for use in type II diabetes, obesity, and related metabolic disorders. Antagonists to FGF23, a key regulator of phosphate homeostasis, are being developed as the first causative therapy for patients with renal phosphate wasting disorders to block the action of excess FGF23 in these patients, and are also considered for treating cardiovascular disease in chronic kidney disease. FGFs carry out their diverse functions by binding, dimerizing, and thereby activating the FGF receptor (FGFR) subfamily of RTKs. The classic paracrine FGF ligands require heparin sulfate (HS) as a cofactor for activating FGFRs, whereas the hormone-like FGFs depend on Klotho coreceptors. HS- or Klotho-assisted FGF-FGFR dimerization enables the trans- phosphorylation on cytoplasmic kinase A-loop tyrosines to activate the autoinhibited ?catalytically-repressed? FGFR kinases, and therefore is a mandatory step in FGF signal transduction. The tremendous biomedical significance of FGF signaling has been the driving force behind the studies on the structural basis of this signaling system, which my laboratory has been spearheading since inception of this grant in 2000. Building upon the achievements of the previous grant cycles, in this competing renewal we propose three Specific Aims to address imminent knowledge gaps in FGF signaling that will have a significant and persistent impact on FGF/RTK signaling and its therapeutic targeting. Aims I & II will unravel the mechanisms by which Klotho coreceptors assist endocrine FGFs to dimerize and activate their cognate FGFRs, and hence pave the way for the discovery of novel therapeutics for major metabolic diseases, such as type II diabetes, obesity, and chronic kidney disease, which pose some of the biggest threats to public health and worldwide economy. Aim III will exploit a unique gain-of-function mutation in FGFR to solve the longstanding paradox of how unphosphorylated catalytically-repressed RTKs perform the initial A-loop tyrosine transphosphorylation to become activated. Our discovery of an unprecedented ?induced fit? asymmetric dimerization of FGFR kinases, which triggers A-loop tyrosine phosphorylation by overcoming an autoinhibitory charge repulsion, will advance our comprehension of signal transduction by majority of RTKs that rely on A-loop tyrosine transphosphorylation as the activating step. This Aim will also provide the details of how this pathogenic mutation hijacks the physiological mechanism underlying A-loop tyrosine phosphorylation to confer gain-of-function on the kinase.